JP 2000-271117 A discloses a method of detecting a boundary of a blood vessel wall using ultrasonic waves. According to this method, assuming that a blood vessel has a normal structure, a displacement and a diameter of the blood vessel, a thickness of a blood vessel wall and the like can be measured based on a maximum peak value and a second peak value of a brightness signal in image data obtained based on ultrasonic waves reflected by the blood vessel.
Further, Japanese Patent No. 2889568 describes a method of detecting an IMT (Intima-Media Thickness: a thickness from an inner membrane to a middle membrane) value of a blood vessel wall (carotid artery) using ultrasonic waves. According to this method, assuming that a blood vessel has a normal structure, an IMT value of a blood vessel wall (carotid artery) is measured based on a maximum peak value and a second peak value of a brightness signal in image data obtained based on ultrasonic waves reflected by the blood vessel.
In the above-described conventional technologies, however, since the brightness signal of image data is utilized for analyzing a structure of the blood vessel wall as a target of the measurement, if a brightness of an inner membrane of the blood vessel wall as the target of the measurement is low, there is a problem that the displacement and the diameter of the blood vessel, the thickness of the blood vessel wall and the like cannot be measured correctly. In addition, they are based on the assumption that the structure of the blood vessel wall as the target of the measurement has a normal structure. Therefore, there is a problem that the displacement of the blood vessel or the like cannot be measured correctly if there is a local disease such as an atheroma in a blood vessel as a target of the measurement.
Similarly, in the above-described conventional technologies, the brightness signal of image data is utilized for analyzing a structure of the blood vessel wall as a target of the measurement of an IMT value. Therefore, there is a problem that the IMT value of the blood vessel wall cannot be measured correctly if a brightness of an inner membrane of the blood vessel wall as the target of the measurement is low.